Massachusetts, Summer, 1951: When a boy drowns in the Charles River, a family is devastated, and the town of Norumbega is changed forever. Eight years later, Frances Adams lives in the shadow of her dead brother, Mac. At her window dresser job, she befriends model Gwen, whose resemblance to Frances inspires her to reinvent herself. When Gwen vanishes into the river, Frances becomes obsessed with her memory—and transforms herself into Gwen's likeness. An investigation begins, and the police question three men. Frances and friend Iris follow them to Riverside Park, where one of the men—the suspicious Harris—mistakes Frances for Gwen. Intrigued by the idea of haunting him, she continues her secret pursuit, immersing herself in the case. As the mystery unravels, shocking revelations about its connection to a long-past family tragedy come to the surface. Set in the haunting atmosphere of 1950s and 1970s New England, In the Vanishing Hour weaves suspense and mystery into a story about loss, identity, and secrets best left untold.